Options to Control Powdery Mildew Evaluated

Annual field day looks at efficacy of fungicide treatments

Plant pathologist Dr. Doug Gubler from the University of California, Davis, discusses results of 2013 fungicide field trials for powdery mildew control in a Chardonnay vineyard in the Clarksburg AVA.

Courtland, Calif.—Grapegrowers, vineyard managers, consultants, pest control advisors (PCAs) and ag chemical suppliers had the opportunity to inspect the results of 90 different fungicide spray treatments to control powdery mildew Aug. 7 as part of a University of California Cooperative Extension (UCCE) vineyard field trial conducted by the University of California, Davis, Department of Plant Pathology.

UC Davis plant pathologist Dr. Douglas Gubler, an expert on grape fungal pathogens and diseases and a frequent lecturer at industry meetings, oversees the annual research trial that has been conducted in the same Chardonnay vineyard for 28 years in cooperation with grower John Baranek of Herzog Ranch in the Clarksburg American Viticultural Area.

As Gubler explained, this vineyard near the Sacramento-San Joaquin River Delta is an excellent test site, with powdery mildew (Erysiphe necator) disease pressure each season. “This site has high mildew pressure because the weather is generally cool, and it’s next to a body of water (Snodgrass Slough) that provides cooling from the wind blowing across the water and into the vine canopies,” Gubler said. The 2013 season had moderate pressure with powdery mildew (PM) showing up in early to mid-May, a little later than in 2012.

“When the disease shows up by April 15, it’s usually a severe pressure year,” Gubler said.

The 90 treatments tested in 2013 included synthetic, biological and organic fungicides, in different combinations, and with varying application rates and timing. Applications were made beginning in early April, and final treatments were applied the week of July 15. Leaf removal was done in the cluster zone in late May and early June to open the canopy to air movement and better expose clusters for spray coverage. Leaf removal assists with control, because PM spores are thin-walled and negatively affected by UV light.

The treatment applications were made using a “Nifty Fifty” hand-gun sprayer by UC Davis staff research associate Ian Bay and graduate student researcher Trang Nguyen. A Metos weather station located within the trial block monitored daily temperature and precipitation conditions, and researchers utilized the Gubler-Thomas Powdery Mildew Risk Index to calculate disease pressure and schedule spray applications and intervals. Bay said spraying can generally be stopped when the grapes reach véraison in the range of 8° to 12° Brix.

The annual trial involves cooperators from ag chemical supply companies who evaluate new and proprietary fungicides before going to market, try registered products in different combinations, or use the trial to test and improve application protocols. Bay said, “These cooperators give us different treatment protocols they want to evaluate to see how materials work at different rates, different intervals, in different concentrations or with adjuvants to improve spray contact.” The trial is set up each season based on the number of treatments involved. Bay, who has worked on the field trial the past five years, said as many as 125 total treatments have been tested in a season.

Integrated program improves efficacyGubler noted that organic oils such as JMS Stylet Oil (a paraffinic oil), PureSpray and Safe-T-Side, which work as contact fungicides with a smothering or barrier effect to PM growth, continue to be very effective products, particularly when used early in the season to knock back the fungus. One caution with these oils is that they can slightly delay fruit maturity, as the oil will block and shut down leaf stomata to reduce carbohydrate accumulation. Gubler said, “Some winemakers restrict the use of oils in some vineyards, so this is something to discuss with the wineries you provide grapes to.” Other types of natural oil products tested this year included Centurion (cinnamon oil) and Timorex Gold, an oil derived from the tea tree (Melaleuca alterniflora).

Another reason this vineyard is a good test location is because PM here shows resistance to DMI fungicides, the demethylation inhibitors, also known as sterol inhibitors. Gubler noted that DMI resistance is found throughout California grapegrowing regions—here in the Delta and nearby Lodi, and also in the North Coast, Central Coast and the southern San Joaquin Valley. “Powdery mildew shows resistance to DMI fungicides under high disease pressure, but under normal pressure situations they will work,” Gubler said. He advised, “Know what you’re dealing with. You can insert these products into spray timings when there is lower disease pressure.” DMI products in the 2013 trial included Inspire, Luna Experience, Procure, Rally, Rhyme and Topguard.

Other products tested included copper, sulfur and phosphonate-based fungicides. Biological products included Sonata (Bacillus pumilus) and Taegro (Bacillus subtilis). Another natural product tested was Fracture, a plant protein extracted from the genus Lupinus.

A general recommendation is that different classes of fungicides with different modes of action be used alternately during the growing season and from year to year in an integrated program to prevent disease resistance and improve efficacy. Soft chemistry, natural and biological products can be used when disease pressure is low to moderate to practice good integrated pest management (IPM) and still get good disease control.

Mildew can acclimate to high temperatureGubler also provided an update on recent research to reevaluate the high temperature range for powdery mildew growth and how this will likely be used to revise the Gubler-Thomas risk index. Gubler explained: “Our previous thinking was that the fungus, when subjected to high temperatures of about 91°-92° F. would start to die off. We thought if we had two to three days at these high temperatures we could stretch out the spray intervals up to 11 days later. We now have new information that the fungus can acclimate to higher temperatures, and it appears that temperatures must be at least 94° to 96° F before we start seeing a negative effect.”

Field day attendees received a copy of the Preliminary Report for the 2013 field trials, which showed trial plots for each vine row and listed treatments that corresponded to marker flagging on the vines. This allowed comparison of the efficacy of different treatments with each other, and with control vines in each row that had no treatments and had significant PM disease and damage. A final report with results of disease severity and incidence for each treatment will be posted on the website for the UC Davis Department of Plant Pathology within the next two months. Copies of annual PM field trial reports going back to 2003 are currently posted here.